Synth DIY

Schematics and links relating to 'synth diy' and other audio technology.

Circuit Ideas

Provided are some schematics and circuit ideas to complement the wealth of other excellent resources available on the web!

Discrete State Variable VCF

The acrobat file GH_SVF2A.pdf illustrates a discrete VCF design. Discrete designs have the advantage of being cheap, economical in use of transistors, often low noise (depending on transistor) and they can always be adapted to use a different transistor. This design has no opamps in the signal chain and has very good overdrive characteristics. Voltage controlled resonance is added, with an exponential converter to provide the correct response with a linear CV. Signal inputs and outputs are 1Vpp.

Multimode VCF

The acrobat file GH_MMVCF2a.PDF contains a multimode VCF design idea that offers 12db LP/BP/HP and 24dB/Octave lowpass. Resonance is implemented with a reverse log 1kR potentiometer although this might be replaced for something more suitable. The filter is implented with the HM2040 voltage controlled filter module (available from the Shop, however any cascaded four pole (real pole) filter design could be adapted to use this technique. This circuit is not fully tested. Any comments and developments welcome!

Voltage controlled delay line

The PT2399 is a handy little delay chip available from a few places like Small Bear Electronics. To make it more useful why not add voltage control to the delay time? A fiddle is carried out with a voltage controlled exponential current source to give a 'useably' linear delay time CV response. TC1 in the schematic (GHSYN2b Web.pdf) should be a +3300ppm/k type. Q1:A and Q1:B should ideally be a matched NPN pair. LM3046 or SSM2210 (overkill) or just two regular NPNs (BC550 etc.) with their cases glued together with heatsink compound.

In this circuit the bandwidth has been reduced quite substantially from the values given in the PT2399 datasheet. This is for several reasons, namely to reduce noise and aliasing but also to closer mimic the 'sound' of analogue delay lines that had a necessarily small bandwidth. This has an interesting effect on delays with feedback, as the signal becomes progressively modified as the audio passes around the loop. Gain-phase plots of the input filter and output filter frequency responses are included below.